1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to systems and methods for directing and correcting high-power beams of electromagnetic energy.
2. Description of the Related Art
Directed energy weapons and specifically high-energy laser (HEL) weapons are being considered for a plethora of military applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. These weapons generally involve the use of the laser or other source of a high-power beam to track and destroy a target. To achieve mission objectives, directed energy weapons must be accurately steered and optimally focused. Steering involves line-of-sight control while focusing, which with respect to HEL weapons, necessitates wavefront error correction. Currently, wavefront error correction is typically achieved using adaptive optics. The current state of the art in laser beam control adaptive optics requires placing one or more deformable mirrors within the highest intensity portion of the beam path. The conventional deformable mirror is typically a large element with a thin face sheet and a number of piezoelectric actuators. Actuators are located behind the face sheet and are electrically driven to push and pull on the surface thereof to effect the deformation required to correct wavefront errors in an outgoing beam. The size of the active region of the deformable mirror must accommodate the full size of the high power laser beam in the high power Coudé path prior to expansion via an output telescope.
In addition, one or more fast steering mirrors may be used to correct for tilt and direct the line-of-sight. A coarse gimbal may be employed to correct for line-of-sight errors as well. A plurality of wavefront sensors are typically employed along with an aperture-sharing element (ASE). The ASE allows a single shared aperture to be advantageously used for both the low power sensors and the high power output laser beam, ensuring that the path through the atmosphere taken by the high power beam is the same as that taken by the wavefront sensor and that the correction applied to the shared atmospheric path is optimal for the high-power beam.
Unfortunately, the use of delicate optical devices in the path of a high-power beam is problematic. This is due to the fact that the high-power beam will heat and distort the optical element unless the element is actively cooled or has a coating with a very low optical absorption coefficient. The most durable coatings require a high temperature application process. Deformable mirrors are typically coated after the face sheet is bonded to the actuators, which limits the maximum temperature to which the deformable mirror assembly may be exposed without degrading the bond. Therefore, coatings may need to be applied at lower than optimal temperature using more complex coating processes, thereby reducing durability and/or increasing manufacturing cost.
In addition, conventional adaptive optics systems using deformable mirrors are limited in performance. Conventional deformable mirrors systems are limited with respect to the speed at which the mirror drive signals are computed and the reaction speed of the deformable mirror mechanism to correct for aberrations. There is also a limitation with respect to the number actuators that can be used. The number of actuators that may be used determines the resolution or “order” of the mirror. The stroke of the conventional deformable mirror is limited. “Stroke” relates to the amount of mirror surface deflection that may be achieved before either the piezoelectric actuators exceed their dynamic range or the face sheet begins to fail. Further, a conventional continuous face sheet deformable mirror cannot correct for a pathology in the spatial phase pattern, such as a branch point or an abrupt phase discontinuity. A branch point is a “singularity” in a deeply scintillated phase pattern caused by atmospheric turbulence over a long propagation path in which the phase monotonically increases around a zero amplitude point like a corkscrew, thereby requiring an abrupt 2π phase correction within the spatial phase pattern. Abrupt phase discontinuities may be caused by the optical discontinuities between segments of a multi-segment primary mirror.
In U.S. Pat. No. 5,694,408, issued Dec. 2, 1997, Bott, Rice, and Zediker appear to disclose a scheme which allows the deformable element to be placed in the low intensity region between a master oscillator and an array of fiber power amplifiers. The approach is to pre-distort the phase of the oscillator beamlets after separation in a distribution network and before injection into the fiber amplifier array, such that the pre-distortion corrects both the piston error between the individual fibers and optical aberrations in the atmosphere. However, this scheme is practical only with a coherently combined array of single-mode fiber amplifiers, as each fiber channel is correctable in piston only, not high order. Also, this scheme is not applicable to multi-mode laser media such as large core fiber amplifiers or bulk media lasers as contemplated for weapon class HEL devices and may not be scaleable to high power levels due to random, high frequency phase noise caused by pump-induced temperature fluctuations within the fibers.
In U.S. Pat. No. 5,090,795, issued Feb. 25, 1992, O'Meara and Valley appear to disclose several related schemes for using a liquid crystal light valve (LCLV) in a self-correcting adaptive optics system. This approach, however, places the LCLV in the high power beam path and is therefore limited by the damage susceptibility of the liquid crystal material.
Accordingly, a need remained in the art for a fast, large-stroke, high spatial bandwidth or high order system or method for effecting wavefront correction of a high-power beam. Additionally, a need remained for a wavefront correction system or method that would operate modulo 2π, i.e., accommodates an instantaneous 2π phase jump anywhere within the phase pattern.
The need was addressed by U.S. Pat. No. 6,809,307 issued Oct. 26, 2004 to Byren et al. and entitled SYSTEM AND METHOD FOR EFFECTING HIGH-POWER BEAM CONTROL WITH ADAPTIVE OPTICS IN LOW POWER BEAM PATH, hereinafter the ‘Byren’ patent. In the Byren patent, a beam control system and method that utilizes the wavefront reversal property of nonlinear optical phase conjugation to permit incorporation of a liquid crystal Optical Phased Array (OPA) within low power legs of the beam control system is disclosed and claimed. The heart of the cited invention is the use of deformable optical elements in the low power path of a High Energy Laser (HEL) beam control subsystem to correct for atmospheric turbulence, aero-optic effects, and HEL beam path aberrations.
The Byren patent is adapted for use within an enclosure having a flat window. Unfortunately, as is known in the art, in airborne applications, flat windows are problematic with respect to aerodynamic and optical design and operational considerations. That is, flat windows tend to be more fragile, impose considerable operational constraints on the system and are not conducive to high performance aerodynamic operation. Accordingly, several current and contemplated airborne HEL applications specify the use of spherical, or otherwise conformal, exit windows to minimize aerodynamic drag loads on a beam director turret and high order aero-optic aberrations created with flat windows. For tactical aircraft applications, the use of conformal window instead of flat window will greatly reduce the time varying aero optical disturbances of the surrounding flow field.
However, conformal windows add large-stroke, low-order phase distortions due to lensing as well as chromatic aberration effects resulting from index dispersion, and these must be compensated by the beam control system in order to generate high HEL beam intensity on target.
Hence, a need remains in the art for a system or method for a fast, large-stroke, high spatial bandwidth or high order system or method for effecting wavefront correction of a high-power beam from within a turret with a conformal window.